Let me add my vote, as a former Detroiter (lived there for 20 years, roughly concurrent with Mayor Young’s four terms in office), for Detroit/Windsor. I’ve lived more than 50 years of my life in Michigan, always within 40 miles or less of Detroit, and it’s not just Detroiters who see Windsor as a “twin city” with Motown. I think that a previous poster’s mention of the Freedom Festival is more than adequate evidence of the cooperation between the two cities’ governments.
I’d like to add that, when Ontario permitted cities to have casinos, Windsor moved at top speed to get casinos in ASAP. You see, they’re quite as aware as we are of the close economic interrelationship. AAMOF, it was the millions of Michigan dollars that were flowing into the Windsor casinos that Mayor Young used to strongarm Detroit voters, and the state legislature (Detroit is one of the most devoutly religious major cities in the US, with thousands of churches, including some of the “mega churches”, with thousands of members, all of which are opposed to gambling; the MI legislature is dominated by the largely conservative rest of MI) into approving casinos in Detroit.
Now, for other twin cities: 
Port Huron, MI/Sarnia, ON (with massive amounts of freight going back and forth across the bridge there, mostly for the auto plants, but also with lots of recreation - i.e., hunting & fishing - traffic going back and forth. You see, Michigan and Ontario are both generously endowed with lakes and rivers full of fish (careful not to eat too much at a time - remember the mercury!), 
and forests with massive overstocks of deer (especially in MI).
I think I remember someone else mentioning Sault Sainte Marie, MI & ON (just making sure). They’re just across another longish bridge from each other.
I think that most of the residents of the central lower peninsula think of Saginaw/Bay City/Midland as triplet cities; except at rush hours, it’s only about ten minutes on the freeway between any two of the three (I have a slew of cousins living in the area (some in Saginaw, some up in Standish/Bentley), and most of them work in Bay City and shop in Midland.). Bay City is where the automobile plants mostly are, Midland has one of those big outlet malls, and Saginaw has Saginaw Valley State University, as well as the gawdawful Zilwaukee Bridge. :smack:
Likewise, I think that Mackinaw City and St. Ignace (opposite ends of the Mackinac Bridge, and yes, the two are spelled differently on the map, but you’d probably find that a significant fraction of Michiganders spell them both Mackinac) think of themselves as “twins”.
And I think of Ann Arbor/Ypsilanti as twin cities. It’s only because the township that is partly between them refuses to allow the buffer zone to be incorporated that their city limits do not adjoin. Fifty years ago, there was a stretch of land in between that was mostly still farmed. For the last 30 years, it’s all become urbanized, or at least suburbanized, with massive apartment complexes one after another on the main roads between. You see, each of the two cities has a large university; A[sup]2[/sup] has U of Mich, and Ypsi has Eastern Mich U. Each has student enrollments in excess of 25,000, the vast majority of which live off-campus, but not with parents.
Finally, I can offer a pair for Louisiana/Mississippi: Natchez/Vidalia. They are just across the river from each other, and there is no significant disparity in their sizes. The name Natchez is widely recognized because it featured in national history; Vidalia is not (unless you think about sweet onions). 